1. Field of the Invention
The present invention relates to a silicon-nitrogen compound film exhibiting low gas permeability and satisfactory gas-barrier performance, a gas-barrier film using the silicon-nitrogen compound film, and a thin-film device using the gas-barrier film.
2. Description of the Related Art
Currently, various flexible devices are receiving attention. The use of the flexible devices is widely spread, where the flexible devices include, for example, electronic paper, liquid crystal displays, and the electroluminescence (EL) devices.
Basically, the flexible devices have a structure with a thin film of a crystalline semiconductor or metal which is formed in a pattern over a flexible substrate such as a resin substrate. Since the resin substrate exhibits lower gas-barrier performance than the inorganic substrate such as the glass substrate, the resin substrate cannot stop air and water in the air, so that air and water in the air is likely to penetrate through the substrate into the flexible device. When the air or water in the air penetrates into the flexible device, the air or water can affect the element characteristics of the flexible device, and defects or durability reduction can occur. Therefore, it is common to arrange a gas-barrier film or the like in the flexible device for suppressing the penetration of air or water in the air onto the resin substrate. The gas-barrier film is realized by, for example, a silicon-nitrogen compound film such as a silicon-nitride film expressed as SiNx and a silicon-oxynitride film expressed as SiONx, and various silicon-nitrogen compound films exhibiting satisfactory gas-barrier performance are currently being studied.
Japanese Unexamined Patent Publication No. 2004-292877 (hereinafter referred to as JP2004-292877) discloses a silicon-nitride film which is formed by chemical vapor deposition, satisfies the condition 1.05≦x≦1.33 in the composition expressed as SiNx, and has a refractive index of 1.8 to 1.96. JP2004-292877 reports that the disclosed silicon-nitride film exhibits high capability of stopping penetration of water vapor and oxygen. (See paragraph No. 0063 in JP2004-292877.)
Japanese Unexamined Patent Publication No. 2003-105541 (hereinafter referred to as JP2003-105541) discloses a silicon-oxynitride film which is formed by vapor deposition using atmospheric-pressure plasma, and satisfies the condition that the value of x/(x+y) does not exceed 0.95, where the molar ratio between the oxygen atoms and the nitrogen atoms constituting the silicon-oxynitride film is x:y. JP2003-105541 reports that the disclosed silicon-oxynitride film has high flexibility and density, and is resistant to of water penetration. (See paragraph No. 0068 in JP2003-105541.)
Recently, the size of the liquid crystal display has been increasing. In addition, it is known that the element characteristics of the organic EL devices, the organic solar cells, the CIGS (copper indium gallium selenium) solar cells, and the like are likely to be affected by water vapor and oxygen. Therefore, the gas-barrier film arranged in the thin-film device is required to exhibit higher gas-barrier performance. The gas-barrier performance can be evaluated by the water-vapor transmission rate (WVTR). Currently, the WVTR not exceeding 10−6 g/m2/day is required in the organic EL devices. In the fields in which high gas-barrier performance is required, it is common to achieve a desired value of the WVTR by laminating more than one silicon-nitride film through one or more organic layers. Therefore, in consideration of the structure of the device, each gas-barrier film is required to exhibit the gas-barrier performance corresponding to the WVTR not exceeding 10−3 g/m2/day from the viewpoint of downsizing.
However, in JP2004-292877 and JP2003-105541, the gas-barrier performance is not evaluated on the basis of the WVTR. Specifically, the gas-barrier performance is evaluated on the basis of the etching rate in etching with buffered fluoric acid in JP2004-292877, and on the basis of the deterioration in the luminosity of the element in JP2003-105541 by actually arranging the gas-barrier film in the element. In the evaluation performed in JP2004-292877, only the density (compactness) of the film is evaluated, and the other factors are not considered. Therefore, the true permeability of air and water vapor are unknown. In the evaluation performed in JP2003-105541, factors other than the gas-barrier performance are considered to be able to affect the deterioration of the element. Therefore, in JP2003-105541, the gas-barrier performance per se is not evaluated, so that the true permeability of air and water vapor are unknown. Actually, the WVTR value of the silicon-nitride film disclosed in JP2003-105541 has been confirmed as exceeding 10−3 g/m2/day.
Further, the silicon-nitride film disclosed in JP2004-292877 is formed by a special chemical vapor deposition technique, in which raw material gas is catalytically decomposed by using energized heating wires. There is a general tendency for films formed by such a special chemical vapor deposition technique to exhibit higher uniformity in the conditions of various types of bonds between atoms such as the Si—Si, Si—H, and Si—N bonds than the films formed by the other film-formation techniques. Since the conditions of various types of bonds between atoms affect the density (compactness) of the film and the contamination rate, the gas-barrier performance is different according to the conditions of various types of bonds between atoms. Therefore, the films formed by the other film-formation techniques cannot he expected to have the same composition as the silicon-nitride film disclosed in JP2004-292877.